1. Field of the Invention
The present invention is directed to devices and methods to compress two or more adjacent vertebrae across an adjacent bone graft to facilitate fusion of these vertebrae to treat pain produced by pressure from the disks between such vertebrae bulging and resulting in contact with and pressure on the spinal cord and adjacent nerve roots.
2. Description of Related Art
For nearly half a century, anterior cervical discectomy and fusion has been performed for individuals complaining of intractable upper extremity pain due to cervical disc herniation or bone spurs at single or multiple levels. This procedure has undergone several significant modifications since its inception. The introduction of the Smith-Robinson technique of using tricortical iliac crest bone graft, the technique of denuding vertebral endplates of cartilage described by Zdeblick et al., and the present use of cervical plates have all represented significant technical advances which have increased fusion rates and improved patient outcomes. Currently it is possible to expect greater than 85% good or excellent outcomes for individuals with appropriate indications who undergo this surgical procedure.
However, several problems remain. Although fusion rates for one level anterior cervical fusion with autograft (patient's own bone) may approach 95%, these rates decrease significantly for each additional level incorporated in the fusion. Additionally, using autograft bone typically involves the use of a second incision, which significantly increases patient morbidity. Allograft bone (bone from another human) is a viable option, but has considerably lower fusion rates than autograft and is generally not considered a good choice in multiple level fusion surgery.
The use of anterior cervical plates has been credited with increasing fusion rates in multiple level fusions. It is thought that the immediate stability provided by the plate provides a more favorable environment for fusion to occur. The vast majority of plates on the market provide for static stabilization of the vertebral body-graft construct (no compression, no dynamization). More recently dynamic plates have been introduced. These plates provide for passive dynamic compression of the vertebral body-graft construct. This compression occurs post-operatively when the weight of the patient's head loads the construct, allowing for passive compression of the graft to occur. Wolff's law (the concept that bone heals best under compression) suggests that the use of dynamic compression plates should lead to increased fusion rates. However, this has not been found to be the case. Several studies have indicated that dynamic compression plates do not lead to higher fusion rates than static plates. In addition, the possibility of uncontrolled settling over time which may lead to kyphosis (reversal of the normal curvature of the neck) has caused these plates to fall out of favor with many surgeons.
Wolff's law is a well-accepted orthopedic principle, championed and reported in the trauma literature by the Swiss AO Foundation, a non-profit surgeon-driven organization dedicated to progress in research, development, and education in the field of trauma and corrective surgery. Several studies have shown that long bones heal best under rigid compression. This has led to the development of special compression plates that are currently widely used in surgical techniques of open reduction and internal fixation of fractures.
It is believed that there is no plate on the market that truly invokes Wolff's law in spinal fusion surgery by providing rigid static loading of the graft-vertebral body construct. Mechanisms for achieving compression on adjacent vertebrae are known. But, most of these devices either utilize compression across individual screws (risking cut out due to lessened surface area) or attempt to achieve compression prior to the plate being applied (making this a cumbersome technique).